A backlight is conventionally used as an illumination device of a liquid crystal display apparatus. For the backlight, a discharge lamp lighting device having a discharge lamp such as a cold cathode lamp or a metal halide lamp and an inverter circuit for lighting-on the discharge lamp is widely used. Since the discharge lamp needs to be lit with a high voltage, a typical discharge lamp lighting device includes a high-voltage transformer that increases an AC voltage generated from the inverter circuit to a high voltage. The discharge lamp is connected to a secondary side of the high-voltage transformer. The discharge lamp lighting device comprises a protecting circuit that stops the operation of the discharge lamp lighting device when lamp current flowing to the discharge lamp is abnormally high, and the protecting circuit is operated upon detection of overcurrent in the discharge lamp so as to prevent the overcurrent to the discharge lamp (refer to, e.g., Patent Document 1).
FIG. 7 shows a block diagram showing the discharge lamp lighting device disclosed in Patent Document 1. Referring to FIG. 7, in a discharge lamp lighting device 50, an H-bridge circuit 52 for driving a primary side of a transformer 51 is connected to the primary side of the transformer 51, and a discharge lamp 54 is connected to the secondary side of the transformer 51. Further, a logic circuit 53 that generates a signal for controlling the operation of the H-bridge circuit 52 is connected thereto. A protecting circuit 55 receives a signal 56 obtained by dividing an output voltage of the secondary side of the transformer 51. When a voltage of the signal 56 is over a preset threshold, the protecting circuit 55 stops the operation of the logic circuit 53, thereby preventing the overcurrent to the discharge lamp 54.
Upon causing disconnection between a wiring and a terminal on the secondary side of the high-voltage transformer, cutting of the wiring on the secondary side of the high-voltage transformer, disconnection between high-voltage connector terminals for connecting the discharge lamp, the presence of defective wiring of the discharge lamp, or reduction in withstand voltage due to defective covering of a coil of the high-voltage transformer, when an interval (distance) between disconnection positions in the high-voltage wiring circuit is narrow, discharge such as corona discharge or arc discharge can occur at any of the positions. In discharge, there is a problem that sparks due to, e.g., arc discharge, may damage the terminal or parts and smoke and fire may damage the discharge lamp lighting device or liquid crystal device. Therefore, the discharge lamp lighting device having the high-voltage transformer needs to detect discharge such as corona discharge or arc discharge and stop the power supply to the discharge lamp and prevent damage to the discharge lamp lighting device and the liquid crystal display device when the discharge occurs.
However, in the discharge lamp lighting device 50 shown in FIG. 7, even if the discharge such as corona discharge or arc discharge occurs at the partly disconnected position of the wiring on the secondary side of the transformer 51, the discharge lamp 54 is in the lit state and lamp current is constant. Therefore, the voltage of the signal 56 is not over the preset threshold and the protecting circuit 55 is not thus operated. As a consequence, there is a problem that the occurrence of discharge such as corona discharge or arc discharge is not detected.
Then, it is proposed that such a discharge lamp lighting device detects the initial state of corona discharge occurring near the high-voltage transformer and the discharge lamp and protect the circuit (refer to, e.g., Patent Document 2). The Patent Document 2 discloses that, upon the occurrence of corona discharge near the high-voltage transformer or lamp (discharge lamp), an induction pattern for the transformer and an induction pattern for the lamp disposed on a printed circuit board detect the initial state of the corona discharge and protect the circuit, an inverter circuit being protected upon detection of a voltage induced in the induction patterns.
FIG. 8 shows a block diagram showing the discharge lamp lighting device disclosed in Patent Document 2. A discharge lamp lighting device 60 shown in FIG. 8 comprises a rectangular induction-pattern portion 62 for a transformer on the bottom surface of a printed circuit board corresponding to a high-voltage transformer 61, and a rectangular induction pattern portion 64 for a lamp on the bottom surface of the printed circuit board corresponding to a lamp 63. Upon the occurrence of the corona discharge in the high-voltage transformer 61 or the lamp 63, corona-discharge detecting circuits 65 and 66 detect high-frequency voltages induced in the induction pattern portions 62 and 64, and stop switching transistors 67 and 68, thereby protecting the discharge lamp lighting device 60.
Further, such a discharge lamp lighting device is proposed that, upon the occurrence of discharge in the wiring on the secondary side of the high-voltage transformer in the discharge lamp lighting device, the discharge is detected and operation of the discharge lamp lighting device is stopped, it being inexpensive to apply such a discharge lamp lighting device to a large-scale liquid crystal display device (refer to, e.g., Patent Document 3).
FIG. 9 is a block diagram showing the discharge lamp lighting device disclosed in Patent Document 3. Referring to FIG. 9, in a discharge lamp lighting device 70, a discharge lamp 76 is connected to one end thereof on the secondary side of a high-voltage transformer 75. Further, a current/voltage converting circuit 77 for converting lamp current into a voltage and a pattern 78 for controlling lamp current are disposed on the other end of the discharge lamp 76, and a discharge detecting pattern 79 adjacent to and in parallel with the pattern 78 for controlling the lamp current is disposed on the GND on the secondary side of the high-voltage transformer 75. A voltage induced in the discharge detecting pattern 79 is detected and the power supply to the secondary side of the high-voltage transformer 75 is stopped, thereby protecting the discharge lamp lighting device 70.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-168585
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-341775
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-183099